Environmental factors are a key driver in the progression of the aerospace industry. Such factors have resulted in the continual development of gas turbine engines to produce higher power and lower emissions.
Lean-burn combustors, which have low NOx emissions, have been developed for gas turbine engines. These lean-burn combustors operate by increasing the flow of air into the combustor such that the fuel to air ratio is below the level at which NOx is formed. Furthermore, the fuel is burnt at a lower temperature, again reducing the formation of NOx. However, these conditions are also conducive to the occurrence of thermoacoustic instabilities within the combustor. Such instabilities create pressure waves which cause vibration in the gas turbine engine producing undesirable noise and further may result in mechanical failures.
To reduce thermoacoustic instabilities it is known to use Helmholtz resonators. A Helmholtz resonator is a hollow container, typically a sphere, with a neck having an open hole. When air is forced into the container, the pressure inside increases. When the external force pushing the air into the container is removed, the higher-pressure air inside will flow out of the container. However, the inertia of the body of air in the neck causes the pressure in the container to be reduced below the ambient pressure, thus causing the body of air to be drawn back into the container. The body of air oscillates into and out of the container with decreasing amplitude. The amplitude of the oscillation is greatest at the resonant frequency of the Helmholtz resonator.
In our published European patent application EP1669670A1 various Helmholtz resonators have been placed around the circumference of the combustor as for example shown for the inner annulus in FIG. 1. The annular combustion chamber 10 composes an inner annulus 2 and an outer annulus 4. The inner annulus 2 comprises seven Helmholtz resonators 6 spaced around the circumference of the inner annulus 2. A similar distribution of Helmholtz resonators 6 connected to the combustion chamber could be placed within the outer annulus 4.
The thermoacoustic instability in annular combustion chambers creates a pressure wave which spins circumferentially through the combustion chamber. A Helmholtz resonator tuned to the frequency of the wave creates an acoustic node (zero pressure fluctuation) at the circumferential position of the Helmholtz resonator. Consequently, the spinning mode shape is locked in position and a standing wave is present with known positions of anti-nodes (points where the pressure fluctuation is at its maximum).
Thermoacoustic instabilities may arise at different stages in the gas turbine engine operating, range. As the conditions in the combustion chamber (for example temperature) vary during the engine operating range, the frequency of the pressure wave also varies. To account for this, several, groups of the Helmholtz resonators 6 are tuned to different frequencies. This can lead to a large number of Helmholtz resonators at various frequencies around the combustion chamber.
The maximum absorption of a resonator is obtained when it is located at a pressure anti-node. Conversely, no damping is provided at pressure nodes. The Helmholtz resonators 6 are therefore arranged so that additional resonators of the same frequency tuning are included at 90 degree and 270 degree spacing from the Helmholtz resonator 6 which defines the node of the pressure wave. This ensures that all Helmholtz resonators apart from one are located in the vicinity of the anti-nodes to maximise damping efficiency.
However, by distributing additional Helmholtz resonators tuned to the same frequency around the circumference of the combustion chamber it is not known a priori which Helmholtz resonator generates the pressure node by locking the spinning wave, since the position of the node could be at any of the Helmholtz resonator positions. This can lead to a compromise in positioning the Helmholtz resonators and hence a reduction in damping. Furthermore, by using a large quantity of Helmholtz resonators the position of each of the Helmholtz resonators is compromised since not all Helmholtz resonators are located in the direct vicinity of the pressure anti-node. This reduces the amount of acoustic damping provided by the Helmholtz resonators. In addition, the large number of Helmholtz resonators adds weight to the combustion chamber. Moreover, space constraints may not allow the installation of large quantities of Helmholtz resonators on the combustion chamber due to their blockage of the external air flow.
The present invention provides a duct with a Helmholtz resonator configuration which overcomes sonic or all of the above identified problems.